The size reduction performance of a pneumatically driven single-vessel pharmaceutical processor is described. Two materials were considered, namely commercially available granular lactose (CAGL) and high-speed mixer granulator lactose (HSMGL). In both cases there is an initial phase in which the particle diameter is reduced rapidly. The rate at which the size reduction decreases is reduced with time. The final particle diameter was found to be dependent on the material being processed. In the case of CAGL, the minimum size achieved was 150 μm, while in the case of the HSMGL the final particle diameter was 100 μm.